The method according to the present invention avoids overloading the nodes or equipment of the network. In particular, the method is designed to control call traffic, but it can also be used in connection with other kind of traffic, e.g., to control data packets transmitted in a packet network. In the following, however, a telephone network where the traffic consists of calls made by the subscribers of the network is used as an example.
A so-called call gapping method (this name is used in several international standards, such as CCITT Blue Book, Recommendation E.412, .sctn.3.1.1.2, and Recommendation Q.542, .sctn.5.4.4.3) is a control method based on the rate of the traffic, and in this method the amount of traffic, e.g., number of calls, is restricted so that at most a certain number of calls per one time unit are allowed to pass through. This kind of method is described e.g. in U.S. Pat. No. 4,224,479, as well as in the standards mentioned above.
Equipment operating according to the call gapping method can be, e.g., a gapping gate 10 of the type shown in FIG. 1 in which the gate has one input, indicated by IN, and two outputs, indicated by PASS and GAP. A certain predetermined restriction parameter U, which corresponds to a certain number of calls per one time unit (e.g. calls per second), is stored in the gate. The incoming calls are conducted to the input IN of the gapping gate, and the accepted calls are forwarded from output PASS. The gapping gate restricts the frequency (rate of occurrence) of the calls so that the amount of accepted traffic per one time unit is at most the same as the above-mentioned restriction parameter U (calls per second). When the amount of incoming traffic per one time unit exceeds U, the gapping gate conducts some of the calls to output GAP so that the maximum rate of traffic outputted from gate PASS is U. Calls output from GAP can be processed further in many ways, but since the processing of these calls does not fall within the scope of the present application, it will not be described in greater detail herein.
In the aforesaid U.S. Pat. No. 4,224,479, a gapping gate operates such that the calls are given the shortest possible interval, e.g., 0.1 sec. between two consecutive calls. (The restriction parameter stored in the gate can also define the shortest accepted time interval I, which is called a gapping interval, between two consecutive calls. In principle, this is same thing, since the parameters concerned are inverse values of each other, i.e. U=1/I). The gapping gate stores the departure time of the last accepted call. If the difference between the arrival time of a new call and the stored departure time is smaller than the above shortest possible time interval, the call will be rejected. But if the difference is at least equivalent to the time interval, the call will be accepted and the departure time of the last accepted call will be updated such that it corresponds to the current time.
The operation of such a call gapping method is illustrated in FIG. 2. When the amount of traffic offered by the network (represented by the horizontal axis) is smaller than the above maximum U, no restriction takes place. When the amount of offered traffic exceeds this value, the gapping gate rejects some of the calls (conducting them to output GAP), whereby the amount of traffic forwarded (represented by the vertical axis) is U. The ideal situation is indicated by a dotted line and the actual situation by a continuous line. In practice, the characteristic curve (continuous line) describing the operation of the gapping gate is a smooth approximation of the partly linear characteristic curve (dotted line) of the ideal situation.
The drawback of the method described above is that because of the non-ideal restriction performed by the gapping gate, even a large number of calls that could be forwarded are rejected. This means that a gapping gate operating in accordance with the method is not able to produce a very good approximation of the situation indicated by a dotted line in FIG. 2, i.e., a situation where the gapping gate operates ideally. In actual simulations, it has been observed that when the amount of traffic reaches the allowed maximum U, the gapping gate can conduct up to about 40% of the calls to output GAP, although an ideally operating gapping gate should still be accepting all calls. From the point of view of the network operator, non-ideal operation of this kind means that the network operates at reduced capacity with respect to the resources and that no income is obtained for such a large percentage of calls.
That the gapping gate is not able to approximate an ideal situation any better than this is due to the fact that the traffic is not distributed evenly on the time axis but occurs in bursts in which momentary traffic density can be very high.
An object of the present invention is to overcome the above drawback by providing a new kind of control method that enables better approximation of an ideal gapping gate. This objective is achieved with the solutions of the invention.